1. Field of the Invention
The present invention relates to a liquid crystal display device for displaying an image and a method for production thereof. A liquid crystal display device according to the present invention is applicable to a flat display device such as a liquid crystal panel to be used in a projection television, a personal computer, and, in view of a shutter effect thereof in particular, a display board, a window, a door, a wall, and the like. In particular, the liquid crystal display device according to the present invention is applicable to a device in which a thin substrate or a film substrate is used.
2. Description of the Related Art
As display devices utilizing liquid crystal, there have been realized display devices of a TN (Twisted Nematic) mode or an STN (Super Twisted Nematic) mode, which utilize electrooptical effects of nematic liquid crystal. There have also been proposed ferroelectric liquid crystal display devices, in which ferroelectric liquid crystal (FLC) is used, as display devices with higher potentials than the above-mentioned display devices. A ferroelectric liquid crystal display device has such properties as rapid response and memory functions, which a nematic liquid crystal display device does not have. Therefore, ferroelectric display devices are drawing attention, especially as high-resolution liquid crystal display devices and as large area liquid crystal display devices.
However, a ferroelectric liquid crystal display device utilizes, for electrooptical switching, spontaneous polarization occurring at a smectic C.sup.* phase (SmC.sup.* phase), at which the liquid crystal is closer to being a crystal than at the nematic phase, a phase which is used in nematic liquid crystal display devices. Therefore, ferroelectric liquid crystal display devices have the problem of poor shock resistance because of the regular crystal structure of the smectic C.sup.* phase. It is also difficult, in a ferroelectric liquid crystal display device, to uniformly align liquid crystal molecules over a large display. Moreover, since switching is conducted only between two states, i.e. states before and after switching of the spontaneous polarization, there is a problem that no transition state can be obtained. This makes any gray scale display difficult.
A method disclosed in Japanese Laid-Open Patent Publication No. 63-264724 aims to solve the above-mentioned problem of poor shock resistance. According to this method, ferroelectric liquid crystal is dispersed in a polymer, a resultant film being subjected to a stretching process. Japanese Laid-Open Patent Publication No. 63-318526 discloses a flexible ferroelectric liquid crystal display device which has high shock resistance. A ferroelectric liquid crystalline polymer in which a ferroelectric liquid crystal compound is fixed to a side chain of a polymer is used in the display device.
However, the above-mentioned polymer dispersed ferroelectric liquid crystal display device, although having improved shock resistance, has a problem that light scatters at the interfaces between the ferroelectric liquid crystal and the polymer owing to the difference in the refractive indexes thereof. As a result, polarized incident light is depolarized, thereby decreasing the contrast of the display device.
On the other hand, the above-mentioned flexible ferroelectric liquid crystal display device in which a ferroelectric liquid crystal compound is fixed to a side chain of a polymer, has a problem that the response speed thereof is slower by two digits, or two orders of magnitude, than that of ferroelectric liquid crystal itself, since liquid crystal molecules are combined with polymer chains in the display device.
Improvement of the response speed of such a ferroelectric liquid crystal display device is achieved by a method disclosed in Japanese Laid-Open Patent Publication No. 4-59890. The method uses a ferroelectric liquid crystalline polymer added with a low molecular weight ferroelectric liquid crystal having a high response speed. However, the method has a problem that properties of the low molecular weight ferroelectric liquid crystal become more dominant as the content of the low molecular weight ferroelectric liquid crystal increases, thereby lowering the shock resistance of the display device.
With a view to achieving displaying in gray scales, there have been proposed an area-based gray scale method, in which gray scales are displayed by lowering the resolution of the display, and a time-sequential gray scale method, in which gray scales are displayed by lowering the frame frequency of the display device. However, in both methods, each pixel is merely displaying either of the two states of white and black instead of actually displaying in gray scales.